CHEOPS observations confirm nodal precession in the WASP-33 system

TL;DR

This study uses CHEOPS observations to confirm nodal precession in the WASP-33 system, measuring orbital obliquity and stellar properties, and providing insights into stellar structure through planetary orbit analysis.

Contribution

First to combine CHEOPS transit and occultation data with stellar analysis to confirm nodal precession and measure the star's fluid Love number in the WASP-33 system.

Findings

Orbital obliquity measured at 111.3 degrees.

Confirmed rapid secular evolution of the impact parameter.

Estimated the star's second-order fluid Love number.

Abstract

Aims: We aim to observe the transits and occultations of WASP-33b, which orbits a rapidly-rotating $\delta$ Scuti pulsator, with the goal of measuring the orbital obliquity via the gravity-darkening effect, and constraining the geometric albedo via the occultation depth. Methods: We observed four transits and four occultations with CHEOPS, and employ a variety of techniques to remove the effects of the stellar pulsations from the light curves, as well as the usual CHEOPS systematic effects. We also performed a comprehensive analysis of low-resolution spectral and Gaia data to re-determine the stellar properties of WASP-33. Results: We measure an orbital obliquity 111.3 +0.2 -0.7 degrees, which is consistent with previous measurements made via Doppler tomography. We also measure the planetary impact parameter, and confirm that this parameter is undergoing rapid secular evolution as a result of nodal precession of the planetary orbit. This precession allows us to determine the second-order fluid Love number of the star, which we find agrees well with the predictions of theoretical stellar models. We are unable to robustly measure a unique value of the occultation depth, and emphasise the need for long-baseline observations to better measure the pulsation periods.